Amplitude modulated radio frequency amplifiers operating in the VHF and UHF frequencies operate at a low efficiency. Current VHF and UHF transmitter RF power amplifiers are relatively poor in converting primary DC power input to RF output energy available for communication. For mobile and portable transmitters, the end result of the low efficiency is a restrictive communications range and, for military applications, a reduced immunity to interfering signals (reduced jam margin). Typically, conversion efficiencies are less then 20%. This means that a radiated 10 watt RF carrier power output signal requires a combined DC input power and RF drive power of at least 50 watts to the transmitter final amplifier section. The 40 watts (or more) that do not appear in the RF output power are lost as heat within the output stage of the amplifier. The inefficiency of the RF power amplifier not only contributes a problem with excessive heat which must be dissipated, but the generated heat also is involved with an attendant reduction in reliability of the RF amplifier. The associated DC power supply for operating the amplifier must be of a sufficient size to supply the wasted power, thereby significantly increasing the total weight of the unit.
Because of the low operating efficiency of the amplifier, there is a significant limitation on the available RF power output which can be produced from limited capacity power source, such as a battery. By limiting the RF power available for communications, the communication range is limited and the transmitted signal is more susceptible to jamming. On the other hand, any increase in the available RF power achieved by increased circuit efficiency will permit greater communications distance and greater immunity to jamming without an increase in power consumption. Or, an improvement in power amplifier efficiency may be used to achieve an increased battery life without a reduction in the RF output power. Since an increase in efficiency results in less heat generation, there also will be an increase in equipment reliability and an opportunity to reduce equipment weight.
The efficiency of an RF amplifier is primarily determined by the power loss in the active device(s) used in the circuit, e.g., a power transistor or a vacuum tube, and by the losses in the passive circuit components , e.g., matching networks, filters, etc. The power loss in an active device is primarily determined by the instantaneous current through and the instantaneous voltage across the device output element. The active device drive signal characteristics usually have a smaller impact on efficiency. Improvements in operating efficiency usually come from improvements in the characteristics of the instantaneous current through and the instantaneous voltage across the active device output element. These improvements are related to the transfer characteristics of the device selected, and to the operating point chosen for the device. The impact of circuit configuration should be secondary in its impact on efficiency. However, practical considerations and specialized designs often do cause the selected circuit topology to influence the amplifier efficiency.
Normally, amplitude modulation for VHF/UHF military transmitters is achieved by audio rate variation of the RF drive signal (envelope modulation of the RF drive) for the power amplifier. Attempts have been made to operate an RF power amplifier at a more efficient operating point by modulating the DC power for the amplifier rather then the RF drive. Early attempts at output envelope modulation by power supply variation have been found to lead to large amounts of incidental frequency modulation. The physical separation of the output amplifier from the low power RF drive system has encouraged prior attempts at power supply modulation to be performed only on the output amplifier. The RF drive was operated at a predetermined level and was held constant while the output amplifier was modulated by power supply variation. The use of a constant RF drive on the modulated amplifier causes a significant RF overdrive during negative modulation peaks. Similarly, the amplifier is significantly under-driven during positive modulation peaks. This causes modulation transfer non-linearity. The excess drive in the negative peaks tends to be coupled into the output signal, but with a major phase offset. The drive feedthrough phase offsets, plus the phase and propagation delay variations in the device as a result of supply modulation, creates a significant angular modulation as a consequence of the supply variation modulation technique and the RF overdrive. Thus, prior power supply modulation efforts to obtain an increased efficiency concentrated on a constant RF drive to the power amplifier stage and experienced significant incidental angular modulation. This discouraged the use of efficient operating point control techniques.